1. Field of Invention
This invention relates generally to adhering a propylene polymer layer to an enamel coated metal surface and more particularly to effecting a heat seal bond between the surfaces.
2. The Prior Art
Easy opening containers are known to the art. These containers are generally formed of metal and are provided with at least one pour opening. The pour opening generally occupies only a portion of the end panel of the container. Heretofore, the pour opening has generally been formed by scoring to define a tear strip. A pull tab is attached to the tear strip, and upon the application of a force, the pull tab is operative to separate the tear strip from the panel along the score line.
Although easy opening containers have been readily accepted by the public, deficiencies still remain in this type of container. One of these deficiencies is that the removable tear strip which is torn from the end panel in the opening of the can has sharp edges, and when thrown on the ground or otherwise improperly disposed of, remains as a nuisance which presents a cutting hazard to the public.
It has been proposed, e.g., U.S. Pat. No. 3,616,047, to replace the metal tear strip with a plastic layered closure member to eliminate the cutting hazard as the removed portion will not have sharp edges. The closure member fabricated entirely from a thermoplastic resin, such as polypropylene or a laminate of the resin and a metal foil such as aluminum or steel is heat sealed to the surface of an enamel coated end panel having at least one preformed opening, the enameled surface of the panel having been coated with a heat activatable, bond promoting layer containing a carboxyl modified polypropylene resin.
Although the plastic containing closures of U.S. Pat. No. 3,616,047 could be effectively bonded to the metal can ends, problems arose in the application of the carboxyl modified polypropylene layer to the enameled metal surface.
When the carboxyl modified polypropylene resin is utilized as an adhesion promoting layer for bonding the propylene polymer layer of a closure member to an enamel coated metal can end, the carboxyl modified polypropylene resin is applied as a dispersion in a volatile organic solvent such as kerosene. The carboxyl modified polypropylene resin used to prepare the dispersion generally has a particle size of 0.1 to 5 microns and is at the present time a relatively expensive resin material. Although only small amounts of the modified resin are required in the adhesion promoting layer, the application of the modified resin in a dilute dispersion, e.g., 10% solids is unsatisfactory as such dilute dispersions do not have the required physical properties such as viscosity and flow out, which are required for the application of the coating dispersion using conventional coating equipment, e.g. as by roll coating. To obtain a coating dispersion having the physical properties necessary for commercial coating application, polypropylene resin powders of approximately the same particle size range, e.g. 0.1-5 microns are incorporated in the dispersion to raise the solids content to about 20% whereby the dispersion has the flow and viscosity characteristics required in commercial coating methods. One drawback to the use of polypropylene resins of such particle size is that the submicron size resin is difficult to manufacture and is presently in limited commercial supply.